Automatic watches comprise a movement fitted with a time base, a gear train synchronized by the time base, an energy accumulator, generally a barrel, powering the time base and driving the gear train, and an automatic mechanism supplying energy to the energy accumulator.
Conventionally, this mechanism comprises an oscillating weight, pivotably mounted on the frame of the movement by means of a bearing, a reverser converting the alternating movement of the weight into a rotational movement in one direction, and a winding train, which is of the reduction train type, driven by the reverser. The oscillations of the weight, generated by the movements of the person wearing the watch, thus drive in rotation the winding train, which cooperates with the barrel to wind its spring.
The oscillating weight is arranged to carry a bearing, for example a ball bearing, which defines an axis of rotation. It comprises a mass member whose center of gravity is shifted with respect to the axis of rotation. The mass member is generally designed so as to generate maximum torque. It is made of a heavy material, frequently gold or platinum in top of the range watches. At its periphery, it includes a sector of inertia defining the important part of its weight, and a plate connecting the sector to the bearing.
The oscillating weight generates torque essentially as a function of the weight of the sector and the position of its center of gravity, with reference to the axis of rotation. This torque is applied to the first wheel set of the winding train via the reverser. The reduction rate of the gear train forming the winding train defines the torque finally applied to the barrel spring. When the person wearing the watch is a calm person, arm movements unpoise the weight and it is the terrestrial acceleration g which defines the torque. If the person is very active, the accelerations encountered can be substantially greater. Currently, winding mechanisms are chosen so as to provide spring winding conditions for a normally active person. Consequently, with a very active person, the barrel spring is greatly taxed and the risk of excessive wear cannot be ruled out. If, conversely, the person wearing the watch is very calm, the spring barrel is not wound sufficiently.